This invention relates to a grinding apparatus for grinding pipe, particularly in preparation for internal coating. The invention also encompasses a method for grinding the internal surface of pipe and of preparing pipe for internal coating.
Tubulars or pipe, particularly oil-field tubular products, are often coated on their internal surface to protect against corrosion. Also, pipe is often reused and usually the used pipe must be recoated prior to reuse. Regardless whether the pipe is new or used, its internal surface must be sandblasted in order to produce the desired anchor pattern for coating. In addition to providing the desired surface for coating, sandblasting is also used for removing defects and foreign material on the internal surface of the pipe. Prior to sandblasting, the new or used pipe is heated in a gas-fired oven to a burn-out temperature in order to burn or incinerate solvents, oil, old coating, and other material that would otherwise interfere with the application of the desired new coating.
A number of problems arise in preparing new and used pipe for internal coating. Although sandblasting is necessary to produce the surface required for coating, the sandblasting process is inefficient for removing certain blemishes or material on the internal surface of the pipe including, mill scale and slivers, or weld seams and weld splatter on electrical resistance welded tubulars. Oftentimes, several passes are required by the sandblasting device in order to prepare the surface. Thus the sandblasting represents a bottleneck in the coating procedure. The inefficiency problem with sandblasting is exacerbated due to recent safety dictated changes in sandblasting. Job safety regulations now require that more expensive materials be used as the abrasive in the sandblasting process.
Aside from the problems with sandblasting, the burn-out process takes a substantial period of time and uses large amounts of energy to heat the pipe, particularly used pipe. The burn-out process also releases hydrocarbons and combustion products into the atmosphere and is, therefore, detrimental to air quality. Also, some defects in used pipe that prohibit reuse can only be found after burn-out, that is, after substantial expense in heating the pipe in the burn-out procedure.